1. Field of the Invention
This invention relates to current measuring devices and, more particularly to a circuit for measuring the magnitude and polarity of a DC current flowing in a line.
2. Description of the Prior Art
Many different types of devices for measuring DC electric current are known and one such device is a magnetic amplifier which is typically used where the DC current being measured is at a high voltage level making it desirable to electrically isolate the measuring instruments from the potentially dangerous effects of the high voltage. Magnetic amplifier sensors typically utilize a plurality of saturable reactors, which may include a saturable core on which the conductor is wound that carries the DC current to be measured. These saturable reactors may influence the bias voltage from a source of AC source by an amount proportional to the measured current. Finally, an indicating device, such as a meter or the like, provides a readable indicia of the measured DC current.
One such device, disclosed in U.S. Pat. No. 3,855,528 issued to H. Brown on Dec. 17, 1974 entitled DC CURRENT MEASURING CIRCUIT, is a measuring circuit for indicating the magnitude of DC current flow through a conductor which includes a voltage generating circuit for creating chopped DC voltages and a magnetic amplifier with two saturable cores. The current to be measured is passed through the command winding to control the current which the voltage generating circuit applies to the control winding. The measuring circuit exhibits a plurality of equilibrium conditions in each of which the magnitude of the control current flow is determined by the magnitude of the input current, thus causing the control current flow to be a continuous DC output indication of DC input current.
A related measuring device is disclosed in U.S. Pat. No. 3,135,911 issued June 2, 1964 to R. Van Allen entitled POLARITY SENSITIVE SATURABLE CORE REACTOR. This measuring circuitry includes a saturable reactor having two cores, and a polarity sensing circuit inductively coupled to the saturable reactor to produce a unidirectional signal responsive to the direction of the measured current which is in turn used as a control winding of the saturable reactor.
Still another similar apparatus is disclosed in U.S. Pat. No. 2,892,155 issued June 23, 1959 to R. Radas et al entitled APPARATUS RESPONSIVE TO DIRECT QUANTITIES. The Radas et al device is concerned with correcting the non-linear condition that occurs within the range of relatively small values of measured current so that for a zero value of the energizing quantity, there is produced an output quantity having a zero value. A transformer essentially acts as a bias winding and one coil links the cores of the magnetic amplifier. The bias winding is effective when energized to render the cores responsive to the polarity of the energizing quantity to thereby render the measuring instrument responsive to both the magnitude and direction of the energy quantity.
Of interest is a discussion of high-speed reset magnetic amplifiers on page 54 of "Tape Wound Core Design Manual" issued by Magnetic Metals Company of Camden, New Jersey (no date). A self-saturating type of magnetic amplifier is disclosed in which AC power causes the magnetic core to saturate and a rectified waveform appears across a load. The AC voltage resets the cores flux level every half cycle so that there is a new flux level from which the gate circuit will start on the next cycle. A differential output type for high stability type operation to actuate hydraulic valves or other differentially operating devices.
The prior art magnetic amplifier type of current sensing device which provides both a magnetic and polarity indication at the output typically required three or more coils on a saturable core or, if only two coils on each core were used, an elaborate bias adjustment circuit is needed to compensate for variations in temperature and AC source voltage driving the cores that would otherwise distort the output linearity.